1. Field of the Invention
The present invention relates mainly to an exposure apparatus used for a photolithographic operation in a process for manufacturing ICs, LSIs, or other types of semiconductor devices.
2. Description of the Related Art
To manufacture semiconductor devices, such as ICs and LSIs, by using photolithography, an exposure apparatus is employed in which a reticle (mask) pattern as an original is directly projected, or reduced at a predetermined proportion and projected, onto a photosensitive material applied to a semiconductor wafer or a photosensitive substrate. Most photosensitive materials applied to wafers have established proper light exposures. In a conventional exposure apparatus, a half mirror is disposed in an illumination optical system, and the amount of exposure light branched by the half mirror is monitored by a photodetecting element (a first photodetecting means). Based on a result of the monitoring, the light exposure is controlled to obtain an appropriate light exposure.
With the recent trend toward microminiaturization of semiconductor devices, excimer lasers that emit light in a far ultraviolet ray range are being increasingly employed as light sources of exposure apparatuses. It has been found, however, that repeated use of excimer laser beams gradually changes the optical characteristics of an illumination optical system, optical components, such as a half mirror, and coating films. This is considered to be caused by changes primarily in transmittance or refractive index of the vitreous materials of optical components and coating films, the changes being attributed to their exposure to the excimer laser beams. Therefore, the ratio of the light amount of an excimer laser beam branched by the half mirror to the light amount of an excimer laser beam that reaches a wafer changes accordingly. If the light exposure control is conducted on the assumption that the aforesaid ratio stays constant, then the difference between an actual light exposure and an appropriate light exposure may exceed a predetermined permissible value.
Normally, the amount of light emitted from an excimer laser, which is a light source, is controlled by regulating a voltage corresponding to energy for each pulse from an exposure apparatus, thereby conducting the light exposure control. As the transmittance of an illumination optical system changes, a relationship between the transmittance and a voltage value of an excimer laser determined based on the amount of a laser beam monitored will change accordingly. As the transmittance lowers, a light exposure that has been reduced due to the lowered transmittance will be obtained in relation to a command value. In order to perform accurate light exposure control, if the amount of light of a preceding pulse that reaches a wafer is smaller than a set value, then it is necessary to increase the succeeding pulse energy. This requires a higher voltage to be applied accordingly. If the voltage deviates from a permissible voltage range, then a desired amount of light cannot be obtained, preventing precise light exposure control from being carried out.
As a solution to the problem described above, there has been known the following method. A photodetecting element (a second photodetecting means) and a light transmitting portion through which exposure light passes to a portion other than a transfer pattern, with a mask resting thereon, are disposed in the vicinity of a wafer. A ratio is obtained of an output of the first photodetecting means that monitors the light amount in the aforesaid illumination optical system relative to an output obtained when exposure light is passed through the light transmitting portion and incident upon the second photodetecting means, while the mask is away from an exposure range, and irradiation to the wafer is OFF. By using the ratio, the sensitivity of the first photodetecting means under exposure is corrected, thereby to perform exposure with an appropriate amount of light. At this time, a relationship between a command voltage applied to a light source that is dependent upon the amount of light emitted from the light source and an output of the first photodetecting means can also be corrected.
However, in a vacuum ultraviolet range of a wavelength of 200 nm or less, in particular, the transmittance of a vitreous material changes with the irradiation time. The amount of change ranges from 0.1 to 0.3% per one cm, and gradually eases after completion of the irradiation. The time constant is extremely long, e.g., a few tens of seconds. The change amount of the transmittance depends on the pulse energy or oscillation frequency of a laser serving as a light source, an oscillation duty (ratio of a burst oscillation ON versus an oscillation OFF time), exposure duration, the transmittance of a mask, and the amount of light incident upon an optical component in a unit time in an illumination range. A marked change in transmittance is observed, especially immediately after exposure is begun. For this reason, a change in the transmittance of a vitreous constituent located between the aforesaid first photodetecting means and a photosensitive substrate surface presents a significant problem from a standpoint of accuracy in light exposure control. Recently, the diameters of the wafers are being increased, resulting in longer replacement intervals of the wafers. Hence, it is difficult to maintain an appropriate light exposure control accuracy when the relationship between the first photodetecting means, the second photodetecting means, and the voltage of a light source is corrected each time the wafer is replaced. In addition, making frequent corrections inevitably leads to a lower throughput.
Accordingly, it is a first object of the present invention to provide an exposure apparatus and an exposure method that each enables a minimized drop in throughput to be achieved and, also a proper light exposure control accuracy to be maintained.
In one aspect, the present invention provides an exposure apparatus that includes a light source, an illumination optical system illuminating an original on which a pattern is formed by the exposure light emitted from the light source, a projection optical system projecting the pattern to a photosensitive object, a first photodetector, disposed in a portion for receiving light from an optical path between the light source and a portion where the original is placed, the first photodetector being used for monitoring an emission light amount from the light source, and a processing system. The processing system obtains information regarding light exposure provided to at least an optical element included in one of the illumination optical system and the projection optical system, estimates a change in transmittance of the optical element on the basis of the information obtained and corrects a proportional coefficient for the light amount detected by the first photodetector and the emission light amount from the light source on the basis of the estimated change of transmittance.
In another aspect, the present invention provides a method for producing devices by use of an exposure apparatus. The method includes steps of illuminating, with an illumination optical system, an original on which a pattern is formed by exposure light from a light source, projecting, with a projection optical system, the pattern to a photosensitive object, receiving light by a first photodetector from an optical path between the light source and a portion where the original is placed, monitoring, by the photodetector, an emission light amount from the light source, obtaining information regarding light exposure provided to at least an optical element included in one of the illumination optical system and the projection optical system, estimating a change in transmittance of at least the optical element on the basis of the information obtained, correcting a proportional coefficient for the light amount detected by the first photodetector and the emission light amount from the light source on the basis of the estimated change of transmittance and developing the photosensitive object with a projected pattern, a circuit device being produced from the developed object. The illuminating step is performed on the basis of the corrected proportional coefficient for the first photodetector.
In yet another aspect, the present invention provides a method for exposing an original and for projecting a pattern formed on the original onto a photosensitive object. The method includes steps of illuminating, with an illumination optical system, the original by exposure light from a light source, projecting, with a projection optical system, the pattern of the original onto the photosensitive object, receiving light by a first photodetector from an optical path between the light source and a portion where the original is placed, monitoring, by the photodetector, an emission light amount from the light source, obtaining information regarding light exposure provided to at least an optical element included in one of the illumination optical system and the projection optical system and correcting a proportional coefficient for the light amount detected by the first photodetector and the emission light amount from the light source on the basis of a change in transmittance. The illumination step is performed by using the corrected proportional coefficient for the first photodetector.